[dlang] Support extention objects in arrays and the `default` field

[tom-tan]

This request contains the following improvements to dlang codegen.

[Implemented] Support extension objects in arrays

The current dlang code-generated parser rejects the extension objects such as cwltool:CUDARequirement in the requirement field. This request is to accept such extension objects in parser level.

Note: Extension fields such as https://schema.org/name as explained in the user guide are already supported in schema-salad-d level without changing dlang codegen.

[Implemented] Support the default field

The current dlang codegen ignores the default field. Therefore schema-salad-d cannot generate the code to set the default value from the declaration. This request is to fix this issue by adding the @defalultValue attribute to the generated field declarations.

[Implemented] Support MapSchema

[Implemented] Support UnionSchema

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.48387% with 9 lines in your changes missing coverage. Please review.

Project coverage is 83.05%. Comparing base (9a92ffc) to head (d23f263). Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
schema_salad/dlang_codegen.py	85.48%	4 Missing and 5 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #825 +/- ## ========================================== + Coverage 83.03% 83.05% +0.01% ========================================== Files 22 22 Lines 4834 4886 +52 Branches 1310 1326 +16 ========================================== + Hits 4014 4058 +44 - Misses 545 548 +3 - Partials 275 280 +5 ```

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[tom-tan]

Done! I confirmed that the D parser generated from the latest schemas for CWL v1.0, v1.1 and v1.2, and schema-salad-d (HEAD) can parse whole CWL documents listed in cwl-d-auto.

I will do some tweaks to make CI passed.

[tom-tan]

Now ready to merge!

[tom-tan]

Thanks!